1. Field of the Invention
The present invention relates to a zoom lens system, particularly to a zoom lens suitable for forming an image of an object located at very close range onto an imaging device, such as a CCD.
2. Description of the Prior Art
A picture input device has been developed by the present applicant, which has a lens system for forming an image of a picture, e.g., a photographic picture on a negative or positive film or a photo-print, onto an imaging device such as a CCD, so as to output a photoelectric image signal to a video device, e.g., a video recorder, a video printer, a CRT display device, a personal computer etc. The picture input device, named "FUJIX PHOTOVISION FV7" (a trademark), for instance, makes it possible to utilize photographs in various ways in connection with various video devices at hand, and thus widens the pleasure of photography.
The picture input device is made compact, and a picture to be imaged, especially a photographic picture, is set at very close range, i.e. 10 mm to 20 mm, from the lens system of the picture input device. In order to permit imaging a partial area of the picture at variable magnification, it is desirable to use a zoom lens system as the lens system of the picture input device. The zoom lens system of the picture input device should work sufficiently for a subject in the very close range of 10-20 mm.
In general, however, increasing the zoom ratio of a zoom lens increases fluctuation of various aberrations and makes it difficult to maintain a good balance of various aberrations. Especially where a subject is to be imaged at very close range, like in the picture input device, astigmatism becomes too great to control, so that the image formed on the imaging device is inevitably deteriorated. Therefore, the zoom ratio has conventionally been limited to 2 or less for a taking lens of the imaging device.